Terahertz (THz) science deals with the region of the electromagnetic spectrum that is frequently defined as ranging from 0.3 to 20 THz (10-600 cm−1, 1 mm-15 μm wavelength). The field of terahertz science is rapidly developing and portends immense promise in fields as diverse as medical diagnostics, space exploration, environmental monitoring, security, manufacturing and pharmaceuticals. This wide range of areas of application derives primarily from two unique properties of THz radiation, 1) its spectral specificity to vibrational and rotational modes of a wide variety of important chemical and biomolecular species, and 2) to its well known penetrating properties through, for example, conventional packaging materials, clothes and plastics. A third property, its wavelength range of 15 μm to 1 mm, also allows for imaging with good spatial resolution.
The development of sources of THz radiation is being pursued along various conventional approaches, including ultra-fast laser pumped photoconductive switches, pumped gas lasers, optical difference frequency generation and parametric oscillation, frequency doubled diodes, quantum cascade lasers. An alternative approach is based on the superconducting Josephson effect that occurs between layers of superconducting materials that are separated by thin insulating non-superconducting materials. When a voltage difference, U, is applied between the superconducting layers, an alternating electro-magnetic wave (referred to as Josephson plasma waves in what follows) arises in the insulating layer whose frequency, ν, is given by ν=KJ U, where KJ=4.84×1014 Hz/V is the Josephson constant. This translates into a voltage of 2 mV for a frequency of 1 THz. The maximum voltage that can be applied to a superconductor without destroying the superconducting state is limited by the size of the superconducting energy gap, which is a material dependent parameter. The CuO2-based high-temperature superconductors display energy gaps of several tens of meV, and the THz-frequency range is accessible with these materials. In particular, highly anisotropic high-temperature superconductors, such as the Bi2Sr2CaCu2O8 derived superconductors, are composed of superconducting CuO2 layers separated by insulating layers, thus forming stacks of intrinsic Josephson junctions. The potential of the Josephson effect in these compounds as an efficient source for THz-radiation has been recognized for some time, and extensive numerical simulations indicated high THz-power. However, so far it has proven very difficult to extract THz-radiation power from high-temperature superconducting sources, the major limitations arising from a) the large mismatch of the wavelengths of the Josephson alternating electro-magnetic field and of free-space THz-radiation, and from b) the typically very small surface areas from which THz emission can occur.